Potassium-based sorbent was prepared by impregnation with potassium carbonate on activated carbon. The role of water and its effects on pretreatment and CO2 absorption was investigated in a fixed bed reactor. K2CO3 could be easily converted into K2CO3·1.5H2O working as an active species by the absorption of water vapor as the following reaction: K2CO3+3/2 H2O→K2CO3·1.5H2O. One mole of K2CO3·1.5H2O absorbed one mole of CO2 as the following reaction: K2CO3·1.5H2O+CO2↔2KHCO3+0.5 H2O. The K2CO3·1.5H2O phase, however, was easily transformed to the K2CO3 phase by thermal desorption even at low temperature under low relative humidity. To enhance CO2 capture capacity and CO2 absorption rate, it is very important to maintain the K2CO3·1.5H2O phase worked as an active species, as well as to convert the entire K2CO3 to the K2CO3·1.5H2O phase during CO2 absorption at a temperature range between 50 ℃ and 70 ℃. As a result, the relative humidity plays a very important role in preventing the transformation from K2CO3·1.5H2O to the original phase (K2CO3) as well as in producing the K2CO3·1.5H2O from K2CO3, during CO2 absorption between 50 ℃ and 70 ℃.